Road levelling machine

ABSTRACT

A machine for levelling a concrete road has a motor driven chassis fitted with a pneumatic hammer unit arranged to cut away any part of the road surface passing below the hammer unit which exceeds a predetermined level relative to the chassis. Operation of the hammer unit is controlled by a device which measures the height of the road surface immediately ahead of the hammer unit during travel of the machine along a road and energises the hammer unit whenever this height exceeds a predetermined value. A second device measures the height of the cut road surface relative to the chassis and increases or decreases the speed of travel whenever the height of the cut road surface is below or above fixed limits. The distance between the hammer unit and front wheels is approximately three times the distance between the hammer unit and the rear wheels which run on the cut road surface. The hammer unit is of the kind having a cutting head connected to a piston which reciprocates in a cylinder automatically on supply of compressed air, but the valve means is arranged so that air is directed to the top of the cylinder in the event of the piston dropping to its lowermost position, thereby holding the piston in this position. The hammer unit thus stops whenever the cutting head passes over a depression in the road.

United States Patent 1191 Clarke [451 May 14, 1974 22 Filed:

[ ROAD LEVELLING MACHINE [75] Inventor: Ronald Albert William Clarke,

Redhill, England [73] Assignee: Klarcrete Limited, Surrey, England Sept. 20, 1972 [21] Appl. No.: 290,468

[58] Field of Search 299/1, 37, 38, 39; 404/84; 125/6, .7; 173/17 [56] 7 References Cited UNITED STATES PATENTS 3,414,327 12/1968 Austin 299/] 2,553,435 5/1951 Briese 299/37 X Swenson 299/38 X Slater 173/17 Primary ExaminerErne st R. Purser Attorney, Agent, or Firm-Browdy and Neimark [57] ABSTRACT A machine for levelling a concrete road has a motor driven chassis fitted with a pneumatic hammer unit ar ranged to cut away any part of the road surface passing below the hammer unit which exceeds a predetermined level relative to the chassis. Operation of the hammer unit is controlled by a device which measures the height of the road surface immediately ahead of the hammer unit during travel of the machine along a road and energises the hammer unit whenever this height exceeds a predetermined value. A second device measures the height of the cut road surface relative to the chassis and increases or decreases the speed of travel whenever the height of the cut road surface is below or above fixed limits. The distance between the hammer unit and front wheels is approximately three times the distance between the hammer unit and the rear wheels which run on the cut road surface. The hammer unit is of the kind having a cutting head connected to a piston which reciprocates in a cylinder automatically on supply of compressed air, but the valve means is arranged so that air is directed to the top of the cylinder in the event of the piston dropping to its lowermost position, thereby holding the piston in this position. The hammer unit thus stops whenever the cutting head passes over a depression in the road.

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1 ROAD LEVELLING MACHINE This. invention relates'to apparatus for cutting away larities on roads which have a camber or roads which are inclined to the horizontal.

Height irregularities in a road surface, particularly undulations which have 'substantially'the same pitch, can cause excessive vibration in vehicles travelling at speed over the road, even though the amplitude of the undulations, that is the height variation, may be very small, for example one-fourth inch, and the pitch of the undulations very large, for example feet. Such undulations occur in concrete roads cast in square or rectangular bays in which the center of each bay is slightly higher than the margins of the bay. Hitherto the-center of each bay has been ground away by rotary grinding tools, but this is a laborious and costly operation.

According to the present invention there is provided a road levelling machine comprising a chassis adapted to travel along a road to be levelled, a power driven hammer unit mounted on the chassis and having a cutting head adapted to engage the road surface, the hammer unit being operable to impart through the cutting head a succession of impact blows to the road so as to cut away the top surface thereof below the cutting head, leading sensing means arranged to detect the difference in height between the part of the road surface below or immediately ahead of the hammer unit in the direction of travel and the portion of the chassis overlying said part of the road surface, and controlmeans adapted to operate the hammer unit in response to detection by the leading sensing means of a height difference below a predetermined value.

The machine preferably includes a plurality of hammer units mounted on a carriage movable along transverse rails on the chassis, the rails extending in directions transverse to the direction of travel of the chassis, and the machineprovided with means for continuously reciprocating the carriage through a distance such that the path of travel of each cutting head overlaps the paths of travel of the adjacent cutting heads.

The machine of the invention also preferably includes power driving means operable to propel the chassis along the road, trailing sensing means mounted on the chassis and arranged to detect the difference in height between the part of the road surface immediately to the rear of the hammer unit in the direction of travel and the portion of the chassis overlying the last mentioned part of the'road surface, and speed control means operable to regulate the driving means to increase or decrease the speed of travel of the chassis in response to detection by the trailing sensing means of an increase or decrease respectively in the last mentioned height difference above or below predetermined values.

The speed control means thus reduces the speed of travel of the machine when the machine is travelling over a bump which is too large to be cut away in a single pass at the original speed of travel. The speed control means is preferably arranged so that the machine will travel at a speed at which the bump is removed, or

at least cut down below a given height, in a single pass of the machine.

The roadlevelling machine of the invention preferably has the ends of the chassis supported on front and rear road wheels, and the hammer unit so positioned on the chassis that the distance between the hammer unit and the front wheels is more than twice, and preferably three times, the distance between the hammer unit and the rear wheels. This arrangement has the advantage that the hammer unit is adjacent the wheels which run on the part of the road which has been levelled by the hammer unit, and any variation in height of the carriage when the front wheels pass over a bump will only be a fraction of the variation in height of the front wheels, this fraction being the ratio of the distance from the rear wheels to the hammer unit to the distance from the rear wheels to the front wheels.

The leading sensing means may comprise a roller arranged to run over the surface being cut, and an output member movable in accordance with variation in height of the roller relative to the chassis, the output member regulating operation of the control means. Alternatively the cutting head of the hammer unit may constitute the leading sensing means, the control means being adapted to operate the hammer unit in response to movement of the cutting head above a predetermined level and render the hammer unit inoperable in response to movement of the cutting head below said level.

According to another aspect of the invention there is provided a fluid-operated hammer unit for imparting a succession of impact blows to a road surface through a cutting head mounted on the hammer unit, the hammer unit comprising a cylinder closed at the ends thereof, a piston reciprocable in the cylinder, the piston having a piston rod extending as a close sliding fit through a bore in the outer end of the cylinder, and valve means operable to feed fluid under pressure from a supply port to each end of the cylinder and exhaust fluid from the other end of the cylinder in response to movement of the piston towards each end of the cylinder so as to continuously reciprocate the piston, the cutting head being adapted to be mounted on the outer end of the piston rod, characterised in that outward movement of the piston and piston rod beyond a predetermined distance effects movement of the valve means into an inoperative position in which fluid from the inlet port is directed to the inner end of the cylinder so as to terminate reciprocating movement of the piston.

A road levelling machine according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a side elevation of the machine,

FIG. 2 is a rear elevation of the machine,

FIG. 3 is a plan view of the machine,

FIG. 4 is an axial section view of a hammer unit fitted with a cutting head, and

FIG. 5 is a diagrammatic view of the pneumatic control circuit of the machine.

As shown in FIGS. 1-3, the road levellingmachine comprises a chassis 10 having a pair of front road wheels 11 and a pair of rear road wheels 12, a carriage 13 mounted on rails 14 on the chassis for reciprocating movement transversely across the chassis, a platen l5 adjustable on vertical rails 16 on the carriage, a number of hammer units 17 fitted on the platen, a motor 18 op- 3 erated by compressed air and drivably connected to one of the rear wheels 12 (the motor is omitted from FIG. 2), leading sensing means 19 controlling operation of the hammer units and speed of the motor '18, and trailing sensing means 20 further controlling the speed of the motor 18.

The chassis comprises two side beams 25, 26 interconnected at the ends thereof by upper and lower front beams 27, 28 and by upper and lower rear beams 29, 30, the junctions between the beams being reinforced by gusset plates 31. The side beams are of the maximum length which is practicable, compatable with the operation of the machine. Each front wheel is mounted on a separate axle frame pivotally mounted for steering purposes on the lower front beam 28,'the two axle frames having steering arms 36 connected by a track rod 37. A steering handle 38 is pivotally mounted on the upper front beam 27 and coupled to the track rod 37 by a link 39 so that the chassis may be steered by pivotal movement of the handle 38. Each rear wheel is mounted on a separate axle frame 40 pivotally mounted on the lower rear beam 30 for.castoring action. The axles frames 40 are adapted to be locked to the chassis with the rear wheels 12 aligned in the fore and aft direction of the chassis, or transverse thereto, by locking pins (not shown) engageable in co-operating apertures in the axle frames 40 and the rear beam 30. The motor 18 is fitted on one of the axle frames 40 and drivably connected to the associated wheel. The motor is shown in FIGS. 1 and 3 but has been omitted from FIG. 2 in order to illustrate other constructional features of the machine.

The support rails 14 for the carriage comprise two angle iron beams welded at the ends thereof to brackets on the side beams 25, 26, the rails 14 extending transversely across the chassis adjacent the rear end thereof and arranged with one flange of each angle iron rail extending horizontally towardsthe other rail. The carriage 13 comprises four angle iron corner posts 46 interconnected by a pair of front cross beams 47, a pair of rear cross beams 48, and side beams (not shown). The front cross beams 47 embrace the front rail 14 and are fitted with rollers 49 arranged to run on the upper and lower surfaces of the horizontal flange of the front rail 14. Similarly, the rear cross beams 48 embrace the rear rail 14 and are fitted with rollers 49 arranged to run on the upper and lower surfaces of the horizontal flange of the rear rail 14. The bottom of the carriage is provided with front and rear channel section beams 50 welded to the corner posts 46 and fitted with rollers 51 arranged to run on guide rails 52 welded to the lower ends of the brackets 45.

The vertical rails 16 for the platen 15 are formed by flanges of the angle iron corner ports 46 of the carriage, and the platen, which is positioned inside the carriage, is provided at the ends thereof with vertical channel section members fitted with rollers 53 arranged to run on the rails 16. The hammer units 17 are arranged in line along a slot in the platen with the units in abutting contact, the units being rigidly secured to the platen.

The chassis is provided with mechanism for reciprocating the carriage on the rails 14 through a distance at least equal to the spacing between adjacent cutting heads on the hammer units. FIG. 2- shows in broken lines the vertical center line of the carriage at opposite ends of its path of travel. The reciprocating mechanism can be of any suitable construction and is not shown in the drawings. The carriage is provided with mechanism for adjusting the height of the platen. This adjusting mechanism can be of any suitable construction and is not shown in the drawings.

As shown in FIG. 4, each hammer unit comprises a cylinder 55 having a working chamber 56, a piston 57 in the working chamber, the piston having a piston rod 58 extending through a bore 59 in the lower end of the cylinder as shown in FIG. 4, a cylinder head 60 bolted to the'top end of the cylinder and closing off the working chamber 56, and a cutting head 61 mounted on the lower end of the piston rod. The cylinder head is provided with a bracket 62 for bolting the hammer unit to the platen 15. The wall of the working chamber is provided with an annular inlet port 63 spaced from the bottom wall 64 of the working chamber by a distance slightly greater than the depth of the piston 57, the inlet port communicating by transverse and axial ducts 65, 66 with a reservoir 67 formed by an annular groove in the top of the cylinder, the groove being closed by the cylinder head 60 which is provided with a pipe 68 for feeding air under pressure into the reservoir 67. The wall of the bore 59 is provided with exhaust ports 70 which open to atmosphere through the outer wall of the cylinder. The piston and piston rod are provided with an axial duct 71 which opens through the top of the piston into the working chamber 56, and the piston rod is provided with transverse inlet ducts 72 intersecting the duct 71 and spaced from the piston a distance such that the ducts 72 are open to the lower end portion of the working chamber 56 when the piston is in the upper half of the working chamber but closed by the wall of the bore 59 when the piston is in the lower half of the working chamber. The piston rod is also provided with transverse exhaust ducts 73 which intersect duct 71 and communicate with the exhaust ports 70 in the cylinder when the piston is in the lower half of the working chamber, but are closedby the wall of the bore 59 when the piston is in the upper half of the working chamber. The lower end portion of the piston rod is provided with a narrow axial bore 75 which communicates with the axial duct 71, and the cutting head 61 is provided with an axial bore 76 co-axial with bore 75 so that air under pressure from the duct 71 can flow through bores 75 and 76 to the working face of the tool and blow away dust cut away from the surface of a concrete road. The piston and piston rod are free to rotate about their axes, and a seal 77 is mounted in a groove in the lower end of the bore 59 to prevent entry of dirt into the bore 59. j

The cutting head 61 is formed with a conical recess 78 and the lower end of the piston rod 58 is tapered and engaged as a tight fit in the recess 78. The lower face of the cutting head is provided with teeth 79 which extend radially outwards from the axis of the head.

In operation, the hammer unit is in a vertical position as shown in FIG. 4, with the cutting head 61 in contact with the concrete to be cut. When the cylinder 55 is at a height above the concrete such that the piston 57 is spaced from the bottom wall 64 of the working chamber when the cutting head is resting on the concrete, then supply of compressed air to the pipe 68 causes the piston, piston rod and cutting head to reciprocate and thereby impart impact blows to the concrete. The air flows from the pipe 68 into the reservoir 67 and thence through ducts 66, 65 and the annular port 63 into the lower end of the working chamber below the piston. The pressure of air acting on the underside of the piston forces the piston upwards, the air in the working chamber above the piston being expelled to atmosphere through the ducts 71, 73 and the exhaust ports 70. When the piston rises to a height such that the ducts 73 are closed by the wall of the bore 59, thereby preventing escape of air from the working chamber, and the upper edge of the ducts 72 are above the bottom wall 64 of the working chamber, then the air flowing into the lower end portion of the working chamber flows through the ducts 72, 71 into the upper end of the working chamber. The pressure of air acting on the top face of the piston is then equal to the pressure of air acting on the bottom face of the piston, but since the area of the top face is greater than the bottom face, the air will exert a downward thrust on the piston. When the piston is forced down by the air to a height at which the ducts 72 are blanked off by the wall of the bore 59 and the ducts 73 communicate the exhaust ports 70, the air in the upper portion of the working chamber will exhaust to atmosphere through ports 70 and the air in the lower portion of the working chamber will again thrust the piston upwards.

If however the piston 57 drops to a level at which the top surface of the piston is below the upper edge of the inlet port 63, air from the port 63 will flow directly into the upper end portion of the working chamber and hold the piston stationary with the cutting head resting against the concrete. Thus the hammer unit only operates when the concrete is above a predetermined level relative to the cylinder 55, so that the hammer unit stops whenever the concrete is cut away below a given depth or whenever the hammer unit is moved over a depression in the concrete.

All the hammer units 17 are supplied with compressed air from a common line 85 (FIG. 5) fitted with a control valve 86 regulated by the leading sensing means 19, and the air-operated motor 18 (FIG. 3) is fed with compressed air from a line 87 fitted with a first speed control valve 88 regulated by the leading sensing means 19 and a second speed control valve 89 regulated by the trailing sensing means 20. The leading sensing means 19 comprises a two-armed lever 90 pivotally mounted to the chassis of the levelling machine, one arm of the lever 90 being fitted with a roller 91 arranged to run on the concrete surface of the road immediately ahead of the hammer units and the other arm of the lever 90 being formed with a cam 92 cooperating with two pneumatic switches 93, 94. The switch 93 is connected between an air supply line 95 and a line 95' connected to the valves 86, 88 and the switch 94 is connected between an air supply line 96 and a line 96' connected to the valve 86. The switches 93, 94 are of conventional construction and comprise a spring loaded plunger fitted with a roller arranged to engage the cam 92, each switch being arranged to connect its two air lines when the plunger is depressed against the action of the spring by the cam 92, and to close the air supply line and connect the other line to exhaust when the plunger returns to its initial position upon disengagement with the cam 92. The trailing sensing means comprises a two-armed lever 100 pivotally mounted on the chassis of the levelling machine. one arm of the lever 100 being fitted with a roller 101 arranged to run on the cut surface of the concrete road immediately to the rear of the hammer units and the other arm of the lever being formed with a cam 102 co-operating with a pneumatic switch 103. The switch 103 is connected between an air supply line 104 and a line 104' connected to the valve 89. The switch 103 is of the same construction as the switches 93, 94, and is arranged to connect the two lines 104, 104' when its plunger is depressed by the cam 102, and to close the line 104 and connect line 104' to exhaust when the plunger returns to its initial position upon disengagement with the cam 102. The valve 86 comprises a cylinder 105 having a piston 106 formed with an annular groove 107 at the center thereof, the wall of the cylinder being formed with two ports 108, 109 spaced axially on the cylinder and arranged so that the two ports communicate with one another via the groove 107 in the piston when the piston is abutting one end of the cylinder, while the port 109 is blocked by the piston when the piston is abutting the other end of the cylinder. The two ports are connected to different sections of air line 85, the air line 96' is connected to one end of the cylinder and the air line 95' is connected to the other end of the cylinder, the arrangement being such that air under pressure supplied through line 95' forces the piston against one end of the cylinder so as to put the two ports 108, 109 in communication and thereby open line 85, while air under pressure supplied through line 96 forces the piston against the other end of the cylinder so as to block the port 109 and thereby close the line 85.

The air line 87 for feeding air under pressure to the motor 18 has two by-pass lines 115, 116 which by-pass the section 87 of line 87 fitted with the valve 88, the valve 89 being fitted in the by-pass line 116. The valve 88' consists of a cylinder 118 having ports in the wall thereof connected in the line 87, a piston 119 movable in the cylinder between a blocking position in which it closes the line 87 and an open position in which it is spaced from the ports, and a spring 120 disposed between the piston and one end of the cylinder and urging the piston into the blocking position. The air line 95' opens into the opposite end of the cylinder so that air under pressure fed into the cylinder through line 95 will force the piston into the open position.

The valve 89 is similar in construction to valve 88, and consists of a cylinder having ports in the wall thereof connected in the line 116, a piston 126 movable in .the cylinder between a blocking position in which it closes the line 116 and an open position in which it is spaced from the ports, and a spring 127 disposed between the piston and one end of the cylinder. In valve 89 however the spring 127 biases the piston into the open position, and the air line 104' opens into the opposite end of the cylinder so that air under pressure fed into the cylinder through line 104' will force the piston into the blocking position.

To level a concrete road with the levelling machine, the platen 15 is first adjusted to a height at which the cutting heads will engage the parts of the road to be cut away and raise the pistons of the hammer units above the bottom walls 64 of their working chambers. The carriage 13 is continuously reciprocated across the chassis, and the levelling machine is driven by the motor 18 in the forward direction across the concrete road to be levelled, the machine being guided by an operator holding the steering handle 38. When the carriage is passing over a part of the road which is level, that is a part of the road lying the plane which passes through the points of contact of the front and rear wheels with the road, the rollers 91, 101 are in their lowermost positions with the cam 92 engaged with the switch 94 and the cam 102 disengaged from the switch 103. The lines 95, 104' are then connected to exhaust and the line 96 connected to the supply line 96. Under these conditions the piston 106 of the valve 86 is in the left hand position, as viewed in FIG. blocking the air supply line 85 to the hammer units which are then inoperative. The valves 88, 89 are in the open position so that air flows to the motor through the line 87' and the two by-pass lines 115, 116 to drive the motor 18 at the maximum speed.

When the roller 91 passes over a raised portion of the road, the lever 90 swings in the counterclockwise direction as viewed in FIG. 5 so that the cam 92 is disengaged from switch 94, resulting in line 96' being connected to exhaust, and cam 92 is engaged instead with switch 93, resulting in line 95 being connected to the air supply line 95. The air pressure in lines 95 forces the piston of valve 86 into its open position so that air is supplied to the hammer units through line 85. The hammer units will not however operate until their cutting heads ride onto the raised portion of the road and lift their pistons to a level at which they are operable, so explained hereinbefore. The air pressure in line 95' also moves the piston of valve 88 into its blocking position so that the motor is fed only by the reduced quantity of air flowing through the by-pass lines 115, 116 resulting in a reduction in the speed of travel of the machine.

1f the hammer units cut away the road to the required level, the roller 101 remains in its lowermost position.

In the event however of the surface of the road still being above the desired level after the hammer units have passed over, the roller 101 will be swung in a clockwise direction, as viewed in FIG. 5, to engage the cam 102 with the switch 103, resulting in the line 104' being connected to the air supply line 104. The air pressure in line 104 moves the piston of valve 89 into its blocking position to prevent flow of air through the bypass line 116, so that the motor is fed only by the air flowing through the bypass line 115, thereby resulting in a further reduction in the speed of travel of the machine.

The levelling machine may of course be provided with separate sensing means 19, and pneumatic control circuits for each hammer unit or groups of hammer units for use in levelling a road having height variations in a direction normal to the direction of travel of the machine.

If a small part of the road is at or below the desired level, any hammer unit passing over this part of the road will of course become inoperable due to its piston dropping down onto the bottom wall of its working chamber, even though adjacent hammer units passing over raised portions of the road are in operation.

Due to the combination of the forward travel of the chassis and the transverse reciprocating movement of the carriage, each cutting head traces out a zig-zag path which overlaps the paths of the adjacent cutting heads. Each part of the road is thus subjected to substantially the same number of impact blows, so that the road surface is cut away to an even depth.

The leading sensing means 19 together with the associated pneumatic control circuit may be omitted from the levelling machine described above, in which case the machine will cut to a level at which each hammer unit ceases to operate due to its piston dropping below the level at which its top surface is below the upper edge of the inlet port 63. It is however not possible to provide such an accurate control over the levelling operation as with a machine fitted with the sensing means 19.

I claim:

1. A road levelling machine comprising:

a chassis mounted on road wheels adapted to travel along a road to be levelled;

power driving means connected to said road wheels for propelling the chassis along the road;

power driven hammer means, mounted on the chassis and having cutting heads adapted to engage the road surface, for imparting through the cutting heads a succession of impact blows to the road so as to cut away the top surface thereof below the cutting heads;

sensing means mounted on the chassis for detecting height irregularities in the road surface along tthe path of travel of the machine; and

speed control means for regulating the speed of travel of the chassis in response to detection by said sensing means of height irregularities in the road surface.

2. A road levelling machine as claimed in claim 1, wherein said speed control means are operable to regulate said driving means to decrease the speed of travel of the machine in response to operation of said hammer means.

3. A road levelling machine as claimed in claim 1 further including hammer control means for operating said hammer means in response to detection by said sensing means of height-irregularities in the road surface.

4. A road levelling machine comprising:

a chassis mounted on road wheels adapted to travel along a road to be levelled; power driving means connected to said road wheels for propelling the chassis along the road;

power driven hammer means, mounted on the chassis and having cutting heads adapted to engage the road surface, for imparting through the cutting heads a succession of impact blows to the road so as to cut away the top surface thereof below the cutting heads, the road wheels mounted on the chassis to the rear of said hammer means being arranged to run on the part of the road surface cut away by said hammer means; sensing means mounted on the chassis for detecting any difference in height between the chassis and the road surface immediately to the rear of said hammer means during travel along the road; and

speed control means for regulating said driving means to increase or decrease the speed of travel of the machine in response to detection by said sensing means of an increase or decrease respectively in said height difference.

5. A road levelling machine as claimed in claim 4, wherein the cutting heads are spaced across the chassis, and further including reciprocating means for reciprocating the hammer means in directions transverse to the direction of travel of the chassis and through distances such that the paths of travel of each cutting head overlaps the paths of travel of the adjacent cutting heads.

6. A road levelling machine as claimed in claim 4,

sensing means of a height difference below a predetermined valuei v 7. A road levelling machineias claimed in claim 6 further including hammer control means for operating said hammer means in response to detection by said additional sensing means'of height irregularities in the road surface; i

8. Aroad levelling machineas claimed inclaim 4 wherein said hammer means comprises:

a cylinder member; a piston member mounted in said cylinder member and having a piston rod projecting out through a bore in one end of said cylinder member, the piston and cylinder members cooperating to form first and second chambers, the volume of said first chamber increasing upon relative movement of the two members towardsa fully extended position and r the volume of said second chamber increasing upon relative movement of the two members towards a fully retracted position; and valve means for feeding fluid under pressure to said chambers and for exhausting fluid from at least one of said chambers so as to cause reciprocating movement between the piston and cylinder members, one of said members being mounted on the chassis and a cutting head being ,fitted on the lower end'of the other member; wherein said valve means includes inlet and exhaust ports controlled by relative movements of the piston and cylinder members, and said ports are arranged sothat, upon rela-.

tive movement of the piston and cylinder members into their fully extended position said first chamber is'connected to the inlet port and disconnected 1 from the exhaust port, whereby the pressure of fluid in said first chamber tends to hold the piston and cylinder members in their fully extended position. 9. A road levelling machine as claimed in claim 8,

v wherein said cylinder is mounted in a substantially upright position on .thechassis. said piston road extending downwardly as'a close sliding fit through a bore in the glower end of said cylinder, and said cutting head is top of said piston into the upper end portion of said cylinder, a transverse in-let duct in said piston rod effecting I communication between the lower end portion of said cylinder and said transfer duct when said piston is in the upper end portion of said cylinder, said transverse inlet duct being closed by the wall of said piston rod bore when the piston is in the lower end portion of said cylinder, and a transverse exhaust duct in said piston rod effecting communication between said exhaust port and said transfer duct. when said piston is in the lower end portion of said cylinder, said transverse exhaust duct being closed by the wall of said piston rod bore'when said piston isin the upper end portion of wall of said cylinder. 

1. A road levelling machine comprising: a chassis mounted on road wheels adapted to travel along a road to be levelled; power driving means connected to said road wheels for propelling the chassis along the road; power driven hammer means, mounted on the chassis and having cutting heads adapted to engage the road surface, for imparting through the cutting heads a succession of impact blows to the road so as to cut away the top surface thereof below the cutting heads; sensing means mounted on the chassis for detecting height irregularities in the road surface along tthe path of travel of the machine; and speed control means for regulating the speed of travel of the chassis in response to detection by said sensing means of height irregularities in the road surface.
 2. A road levelling machine as claimed in claim 1, wherein said speed control means are operable to regulate said driving means to decrease the speed of travel of the machine in response to operation of said hammer means.
 3. A road levelling machine as claimed in claim 1 further including hammer control means for operating said hammer means in response to detection by said sensing means of height irregularities in the road surface.
 4. A road levelling machine comprising: a chassis mounted on roAd wheels adapted to travel along a road to be levelled; power driving means connected to said road wheels for propelling the chassis along the road; power driven hammer means, mounted on the chassis and having cutting heads adapted to engage the road surface, for imparting through the cutting heads a succession of impact blows to the road so as to cut away the top surface thereof below the cutting heads, the road wheels mounted on the chassis to the rear of said hammer means being arranged to run on the part of the road surface cut away by said hammer means; sensing means mounted on the chassis for detecting any difference in height between the chassis and the road surface immediately to the rear of said hammer means during travel along the road; and speed control means for regulating said driving means to increase or decrease the speed of travel of the machine in response to detection by said sensing means of an increase or decrease respectively in said height difference.
 5. A road levelling machine as claimed in claim 4, wherein the cutting heads are spaced across the chassis, and further including reciprocating means for reciprocating the hammer means in directions transverse to the direction of travel of the chassis and through distances such that the paths of travel of each cutting head overlaps the paths of travel of the adjacent cutting heads.
 6. A road levelling machine as claimed in claim 4, further including: additional sensing means mounted on the chassis for detecting the difference in height between the chassis and the part of the road surface below or immediately ahead of the hammer means in the direction of travel; and additional speed control means for regulating said driving means to decrease the speed of travel of the machine in response to detection by said additional sensing means of a height difference below a predetermined value.
 7. A road levelling machine as claimed in claim 6 further including hammer control means for operating said hammer means in response to detection by said additional sensing means of height irregularities in the road surface.
 8. A road levelling machine as claimed in claim 4 wherein said hammer means comprises: a cylinder member; a piston member mounted in said cylinder member and having a piston rod projecting out through a bore in one end of said cylinder member, the piston and cylinder members cooperating to form first and second chambers, the volume of said first chamber increasing upon relative movement of the two members towards a fully extended position and the volume of said second chamber increasing upon relative movement of the two members towards a fully retracted position; and valve means for feeding fluid under pressure to said chambers and for exhausting fluid from at least one of said chambers so as to cause reciprocating movement between the piston and cylinder members, one of said members being mounted on the chassis and a cutting head being fitted on the lower end of the other member, wherein said valve means includes inlet and exhaust ports controlled by relative movements of the piston and cylinder members, and said ports are arranged so that, upon relative movement of the piston and cylinder members into their fully extended position said first chamber is connected to the inlet port and disconnected from the exhaust port, whereby the pressure of fluid in said first chamber tends to hold the piston and cylinder members in their fully extended position.
 9. A road levelling machine as claimed in claim 8, wherein said cylinder is mounted in a substantially upright position on the chassis, said piston road extending downwardly as a close sliding fit through a bore in the lower end of said cylinder, and said cutting head is mounted on the lower end of said piston rod, said first chamber being formed between said piston and the upper end of said cylinder and said second chamber being formed between said piston, said piston rod, and the lower end of said cylinder, and said valve means comprises an inlet port opening into the lower end portion of said cylinder, an exhaust port opening into the bore through which said piston rod extends, a transfer duct in said piston and piston rod opening through the top of said piston into the upper end portion of said cylinder, a transverse inlet duct in said piston rod effecting communication between the lower end portion of said cylinder and said transfer duct when said piston is in the upper end portion of said cylinder, said transverse inlet duct being closed by the wall of said piston rod bore when the piston is in the lower end portion of said cylinder, and a transverse exhaust duct in said piston rod effecting communication between said exhaust port and said transfer duct when said piston is in the lower end portion of said cylinder, said transverse exhaust duct being closed by the wall of said piston rod bore when said piston is in the upper end portion of said cylinder and also when said piston is abutting the bottom end wall of said cylinder by a distance slightly greater than the depth of said piston whereby fluid from said inlet port flows into the upper end portion of said cylinder whenever said piston drops below a level at which the top of said piston uncovers said inlet port and thereby holds said piston against the bottom end wall of said cylinder. 